The composition of the extracellular fluid in the central nervous system (CNS) of neonates is not as well regulated as that of adults, due partly to the incomplete development of homeostatic transport systems in the choroid plexus. Thus, the realization of the main objective proposed, i.e., to delineate the characteristics of several homeostatic transport systems in the choroid plexus, will increase our understanding of how the brain regulates its electrolyte and acid-base metabolism at various stages of development. Information concerning the development of certain transport system (i.e., those for Na,K,H,HCO3, Cl and organic anions) will be obtained by analyzing the effects on ionic transport of alteration in the activity of two transport enzymes in the choroid plexus: Na-K-ATPase and carbonic anhydrase by inhibitors such as ouabain, acetazolamide and amino-reactive agents. On the basis that a substantial change in ionic transport across a cell membrane is reflected as a change in cellular ionic concentration, the general approach will be to ascertain the nature of drug-induced changes in the concentration of a given ion in the epithelium of the choroid plexus. (Cellular ionic concentrations will be calculated from compartmentalization data.) Data for choroid plexus will be correlated with corresponding data for cerebrospinal fluid for the purpose of constructing models for the transport of ions across the blood-cerebrospinal fluid barrier. The neonatal rat brain is a convenient model for studying the ontogeny of transport systems in the choroid plexus since it is thought that substantial changes in the capacity of certain ionic transport systems occur over a relatively short period of time, thus, in vivo studies are to be carried out with rats of various postnatal ages (i.e., 0.5 to 3 wks). The data obtained will enhance our understanding of the role of epithelial cell transport in the CNS in producing a stable microenvironment for the neural cells in both immature and mature mammals.